The present invention relates generally to the field of projection lithography as it is applied to the production of semiconductor integrated circuits components. More particularly, the present invention relates an apparatus and method for creating a bright, uniform source of partially coherent radiation for illuminating a pattern, in order to replicate an image of said pattern with a high degree of acuity.
Lithography refers to a family of techniques for transferring an image rendered on one form of media onto that of another media, typically by photographically "printing" the image. Obvious examples are posters and postage stamps. Similarly, semiconductor microcircuit have been made, for many years, using this technique: silicon substrates upon which these circuits are to be created are coated with a light (radiation) sensitive material chosen for its ability to accurately replicate the desired image, exposed to a source of radiation partially blocked by a mask to render a pattern. Typically, the circuit pattern is rendered as a positive or negative mask image which is then "projected" onto the coated substrate, in either a transmission or reflection mode, depending on the type of optical system being used. The mask is thus imaged on the surface of the coated substrate where the incoming light (radiation) chemically changes those areas of the coating on which the process light (radiation) impinges, usually by polymerizing the coating exposed to the radiation. Depending on the developer (solvent) used the unpolymerized areas are removed, being more soluble in the developer than the polymerized regions, and the desired pattern image remains.
Since this process allows the user to effectively replicate the mask image indefinitely with little additional expense, "projection" lithography has become an essential and powerful tool for manufacturing semiconductor "chips." However, as the drive to place ever greater numbers for components on those chips continues, the need to resolve ever smaller features also continues. In doing so, the diffraction limits of visible light wavelengths have been reached. In order to continue "printing" these features with acuity, using shorter wavelength radiation is necessary. Currently "deep" ultraviolet and "soft" x-radiation (wavelengths from about 300 nm to 60 nm) are now being actively researched. However, the problem of diffraction limited optics remains and the drive to using wavelengths below 300 nm provide only limited advantage: 13 nm radiation, in fact, only provides enough of an advantage to begin printing features of about 0.10 .mu.m.
As feature patterns shrink higher fluence deep ultraviolet (UV) sources become necessary in order to provide sufficient radiant energy per unit area to "print" these features. Because of this, lasers are now used extensively for semiconductor lithography. Illumination by a laser, however, is, by its very nature, illumination by a "bright", point source of highly coherent light and the smaller (relative) features illuminated by these sources are subject to resonance "ringing. Furthermore, in incoherently illuminated optical systems, smaller (relative) features are attenuated due to a fall-off in the modulation transfer function ("MTF").
It is known that by introducing partial coherence into the illumination affects the image quality of printed features, that is, partial coherence can counter the above mentioned attenuation, and properly adjusted, does not add too much "ringing". Providing a source of partially coherent illumination is normally accomplished by underfilling the optical system entrance pupil with Kohler illumination. In other words, the source is imaged into the entrance pupil and this image is smaller than the pupil by a factor of about .sigma.=0.6. This value of .sigma. is a reasonable compromise in order to achieve the desired balance between attenuation of small features and "ringing" in all features. Factors in the range of 0.4.ltoreq..sigma..ltoreq.0.8 could be used as well.
Chip manufacturers have also begun using "engineered illumination" to help print smaller and smaller features. This technique relies upon the use of various "patterns" of illumination including the "under-filled" Kohler disk, annular and quadrapole illumination, and off-axis illumination. Unfortunately, in order to use one or several of these methods, results in reduced condenser efficiency or requires that the illuminator be seriously modified. For example, when a somewhat higher coherence is needed, an aperture is partially closed which reduces efficiency. If either quadrapole or annular illumination is used, a special mask must be made and inserted in the camera pupil plane. This, in turn, blocks light and thereby reduces efficiency.
Off-axis illumination requires that the condenser be disassembled and reconfigured. All of these methods are time consuming, expensive, and less efficient then current technology albeit capable of achieving smaller design patterns.
Finally, the ability to provide a source of illumination which is not only bright but optimized for the features sizes exhibited by the part pattern to be replicated is critical.
A method for quickly changing and "matching," the illumination pattern used in lithographic systems, would be highly desirable.